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Aerial view of the pond before the technical accident

The case study dedicated to Aurul tailings pond illustrates the use of risk analysis for developing a proper risk management program after a severe technical accident. INITIAL LAYOUT Flat land pond Area: 89ha. Volume: 15 mil. m 3 Maximum height of the contour dike : 17-18m.

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Aerial view of the pond before the technical accident

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  1. The case study dedicated to Aurul tailings pond illustrates the use of risk analysis for developing a proper risk management program after a severe technical accident. MONTREAL

  2. INITIAL LAYOUTFlat land pondArea: 89ha. Volume: 15 mil. m 3Maximum height of the contour dike : 17-18m MONTREAL

  3. Aerial view of the pond before the technical accident MONTREAL

  4. The technical accident On January 30, 2000, at 10pm * a breach of approx. 20m, with a depth expansion until the top of the starter dike on the southern side of the pond  * 100.000 m3 of cyanide-contaminated water were released, beyond control MONTREAL

  5. Dike breach after the technical accident MONTREAL

  6. . Brerach closure to stop spillage MONTREAL

  7. TECHNICAL ACCIDENT CAUSES - The faulty design integral recirculation of water - The excessive input of rainwater.massive thawing + rain of 35.7 l/m2 - Lack of adequate monitoring MONTREAL

  8. A preliminary risk evaluation based on numerical indices @allows for a rational rating of constructive measuresA complete quantitative risk assessment @renders evident the efficiency and the benefits of the structural and non-structural measures in terms of risk management. MONTREAL

  9. FAILURE MODES, EFFECTS AND CRITICALITY ANALYSIS Criticality index : IG = CM . PC . DC where: CM - expresses the component share in the failure mechanism; PC - expresses the component failure probability; DC - expresses the extent to which the component failure may be detected in advance. MONTREAL

  10. Criticality index IG for Aurul pond Parameter CM PC DC IG= /component CMPCDC Freeboard 5 4 1 20 Beach width 4 4 1 16 Downstream slope 5 4 1 20 Grain size of dikes 3 4 3 36* Water collecting system 5 3 4 60 *** Drainage system 5 2 4 40** Pond-plant pipes 3 4 2 24 10 MONTREAL

  11. Prioritization of safety measures established on the basis of criticality index IG : • performance of a second decant tower was given priority • effective drainage of the perimeter dike • close monitoring through an adequate system MONTREAL

  12. Failure mechanisms and associated probabilities MONTREAL

  13. 13 MONTREAL

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  15. Increasing safety measures: @ second penstock ; @ supplementary pump unit with a Diesel engine; @ treatment plant for the decant water, 150 m3/h capacity; @ direct discharge of 100 m3/h with pipe treatment. MONTREAL

  16. Failure probabilities Probability of primary events : # cyclical actions - annual probability based on statistic study of annual maximum values # engineering judgment - annual probabilities on the basis of some numerical equivalence Dam breaching failure probability: initial P b = 4.46 x 10-3 with safety measures P b = 1.412 x 10-3 MONTREAL

  17. Consequences global quantification C = β Σ i CG i P e i α i where: CG i - the gravity index of consequence i; Pi - the probability of effective emergence of category of consequence i; α i - efficiency of the mitigation measures β- owner’s capacity to intervene rapidly for the breach closure. MONTREAL

  18. CG Index – gravity consequences i=1 casualties (C) CG1 =106 i=2 effects on the environment (EE) CG2 =106 i=3 economic loss for the third parties (DTP) CG3 =103 i=4 damage to the owner (DD) CG4 =5x102 i=5 effects on the company image (EI) CG5 =102 MONTREAL

  19. Risk management considerations Risk control is ensured by the imposed safety measures, by monitoring the tailings pond behavior and by complying strictly with the operation regulations. The failure probability of 1.4 x 10 –4 is in the range of the tolerable limits for earth dams. - Reduction of more than 3 times of the probable consequences by successive defensive linesis a rare case in the tailings pond field. MONTREAL

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